We've seen the Guardian take people back and forth in time and alter the past creating alternate timelines, but we've never seen that it can transfer people between different already-established timelines.
That's not to say it can't, just that there's no evidence it can.

I already stated the easiest way for him to return to his original universe: Travel back to right before Nero arrived. Close the rift and/or blow up Nero's ship. This makes the 'new' timeline pretty much identical to the old one. Only difference would be that the Kelvin noted an electrical storm in space, but it quickly dissipated without an apparent cause. On to next adventure...

Now you're on the right timeline, and Spock knows a million ways to get back to his time. Slingshot, Guardian, etc. Only reason he didn't do this is because they didn't WANT to reset the timeline, they wanted it left alone for future movies. Not really any "in-universe" explanation that makes sense, they've fixed worse problems before.

Could you please respect what the writers tried to do and leave the film alone please another STXI lover here I'm getting tired of people nitpicking every single bloody detail of the movie for goodness sake it's just a film.

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You'll have to admit that Star Trek Eleven get no worst judgement than every other Star Trek movie and also all the episodes too. It simply is not being singled out. It gets some special notice partial because it is the most resent addition to Star Trek, also because in all honestly it is a deeply flawed movie. Not that the other works don't have flaws, just this one has more across the board.

You'll have to admit that Star Trek Eleven get no worst judgement than every other Star Trek movie and also all the episodes too. It simply is not being singled out. It gets some special notice partial because it is the most resent addition to Star Trek, also because in all honestly it is a deeply flawed movie. Not that the other works don't have flaws, just this one has more across the board.

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Actually, this movie is no more or less flawed than any other Star Trek movie. . . you just don't happen to like this one, and that makes its flaws stand out to you. For instance, let's look at WoK, shall we? How does StarFleet lose a planet? . . . supposedly Chekov had met Khan (even though Koenig wasn't a cast member until the second season - but let's handwave that away). . . why doesn't HE remember that there was an extra planet in that system? I mean, after marooning one of the most dangerous persons in the history of Earth there, you'd think that someone would have put a warning beacon telling people to stay out of the system, or, I don't know, kept a closer eye on what was happening there. . . how can they use their sensors to pick up microbial life and MISS the humans standing less than 5 feet away? What is protomatter. . . it is never explained. . . how does a scientist of Carol Marcus' intelligence not know that her son used protomatter in her own experiment? What star is the Genesis planet orbiting?

I can go on and on, and I can do that for all of the Trek movies. . .if a movie is entertaining enough then people can forgive most flaws. . . obviously, a lot of people thought that Star Trek XI was entertaining. . .your mileage may vary.

I already stated the easiest way for him to return to his original universe: Travel back to right before Nero arrived. Close the rift and/or blow up Nero's ship. This makes the 'new' timeline pretty much identical to the old one. Only difference would be that the Kelvin noted an electrical storm in space, but it quickly dissipated without an apparent cause. On to next adventure...

Now you're on the right timeline, and Spock knows a million ways to get back to his time. Slingshot, Guardian, etc. Only reason he didn't do this is because they didn't WANT to reset the timeline, they wanted it left alone for future movies. Not really any "in-universe" explanation that makes sense, they've fixed worse problems before.

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Once again: Spock didn't have the right. Kirk's entire life has been defined by Nero's tampering. You can't just say "I'm gonna effectively kill you and replace you with William Shatner", and undo 25 years of everyone's lives. It's wrong.

Kills the rest of the argument, though. Either this universe REPLACED the original one, in which case, Spock would be obligated to try and restore his timeline regardless, or this is simply ANOTHER of myriad universes, in which case, it will exist along with every other possible outcome, and Spock's only obligation is to himself, and whether he wants to return home.

I already stated the easiest way for him to return to his original universe: Travel back to right before Nero arrived. Close the rift and/or blow up Nero's ship. This makes the 'new' timeline pretty much identical to the old one. Only difference would be that the Kelvin noted an electrical storm in space, but it quickly dissipated without an apparent cause. On to next adventure...

Now you're on the right timeline, and Spock knows a million ways to get back to his time. Slingshot, Guardian, etc. Only reason he didn't do this is because they didn't WANT to reset the timeline, they wanted it left alone for future movies. Not really any "in-universe" explanation that makes sense, they've fixed worse problems before.

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Once again: Spock didn't have the right. Kirk's entire life has been defined by Nero's tampering. You can't just say "I'm gonna effectively kill you and replace you with William Shatner", and undo 25 years of everyone's lives. It's wrong.

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And 6 billion Vulcans, who would have otherwise lived for 100 years and longer, have been killed by Nero. I'd say that outweighs anything else.

Spock slingshots backwards (in a ship other that the Mega-prise) to a point before Nero arrives when the two universes are still one. Then Spock slingshots forward into the future of the prime universe (his universe) to a point immediately after he left the future, Where the universe branches, he moves into the correct branch.

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Sounds so easy a child could do it.

If time travel was that easy, everybody would be doing it.

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In the Star Trek universe, it is pretty easy. Spock has done it several times himself. So he knows how to do it. But for some reason, he elected not to in this alternate universe.

Neither universe is the Prime Universe, since both have different Stardate systems.

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The writers decide that stardates should make sense now, and it gets used as "proof" that Nimoy Spock isn't from Classic Trek, undermining the whole point of his cameo.

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Then the STXI writers should have done it right to begin with. If they actually expect anyone (well...TOS fans at least) to believe that OldSpock and Nero came from the Trekverse that we know and love, then they should have at least made some effort to connect it to the original. Instead, we got a completely different stardate system and a Kelvin that doesn't fit with the look or feel of the established universe. Unfortunately, the writers apparently thought the "target audience" they were aiming for was too dumb to understand that stardates weren't the same thing as gregorian dates. So yes, that gets used as proof, or at least circumstantial evidence, that OldSpock isn't from the Classic Trek universe.

Saying that saving six billion Vulcans is reason to go back and undo everything is BS. Why? In the Prime future at least as many Romulans and Remans are dead. If you "fix" the timeline they all die again. In the alternate reality, with foreknowledge of the Hobus supernova, all those deaths can still be averted.

Billions die in both universes - probably more in Prime because of the destruction of Remus too. Can you imagine Vulcans wanting their world saved at the expense of two others? Even those of their enemies?

Saying that saving six billion Vulcans is reason to go back and undo everything is BS. Why? In the Prime future at least as many Romulans and Remans are dead. If you "fix" the timeline they all die again. In the alternate reality, with foreknowledge of the Hobus supernova, all those deaths can still be averted.

Billions die in both universes - probably more in Prime because of the destruction of Remus too. Can you imagine Vulcans wanting their world saved at the expense of two others? Even those of their enemies?

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Spock going back, saving the Kelvin, saving Vulcan AND sending a secret message to the Federation and the Romulans to beware of the Hobo star is not helping?

Spock once went back in time to get some whales to save Earth. So he wouldn't have a problem with warning Romulans about the supernova, while he's already in the past.

and he's in a past that he already knows is not going to result in the future he's familiar with, so it's pretty much fair game with whatever changes he wants to make. Only a matter of how much he wants to muck with things.

Kind of a tough spot, actually. Does he just write down a list of big problems and solutions? (Hey, remember that dictator you froze and shot off into space? Might wanna watch out for him in 4 years or so. Doomsday machine, space ameoba, probe, parasites on Deneva, oh, and these things called the Borg...).

How much does he warn them about in order to save lives, how much does he let slide because it'll, for lack of a better word, build character in the heroes?

I've always believed--from the opening shot of the Kelvin--that Star Trek XI took place in an alternate universe from the very start. But I also think the writers left it deliberately vague enough for people who thought it was the same universe--or for people who didn't give a damn either way--to have it their way too.

No matter which route they chose, it wasn't going to please everyone, so it might have been considered something not to really worry about.

But the "alternate universe-from-the-start" approach just works on so many levels for me personally, and really eliminates any continuity complaints I might have had otherwise...